Application of acetylcholine and carbamylcholine to cultured cockroach neurones held under whole-cell voltage-clamp conditions evoked an inward current that was accompanied by an increase in current noise. Fluctuation analysis of the noise revealed the existence of two Lorentzian components in acetylcholine, of corner frequencies 10 ± 0.6 Hz and 116 ± 9 Hz, and one Lorentzian component in carbamylcholine, of corner frequency 35 ± 13 Hz. Single-channel analysis of the unitary currents evoked by acetylcholine or carbamylcholine in neurones held in the cell-attached mode of the patch-clamp technique revealed the presence of two categories of channel events. The large events had mean currents of 4.77 pA with acetylcholine and 5.09 pA with carbamylcholine, and the small events 1.92 pA (acetylcholine) and 1.72pA (carbamylcholine) for a hyperpolarization of 60 mV. The reversal potentials for these currents relative to the resting potential were estimated to be - 70 mV for acetylcholine and - 68 mV for carbamylcholine, and the conductance values calculated from the I/V curves were 37 pS (large) and 19 pS (small) for acetylcholine and 52 pS (large) and 15 pS (small) for carbamylcholine. It is concluded that embryonic cockroach neurones growing in vitro possess two populations of acetylcholine-activated ion channels, and the possibility that one of these represents an embryonic receptor and the other an adult receptor is discussed.

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